In an attempt to define and delineate the human "critical period", we have been longitudinally tracking the monocular visual resolution capabilities (acuity) in esotropic (cross-eyed) infants before and during therapeutic management. Our data indicate that amblyopia (retarded development of visual acuity) begins at six months of age for the congenitally esotropic babies. Prior to any form of therapy the deviating eye shows an arrested development but a seemingly normal development for the straight eye. Short periods (e.g., one week) of occluding the straight eye (the commonly used patch therapy) resulted in an acuity trade-off between the two eyes, i.e., an acuity reduction in the patched eye and an acuity improvement in the deviating eye. Subsequent removal of the patch resulted in a rapid reversion to the previous condition, i.e., the acuity in the previously patched eye increased to above normal levels with a concomitant reduction in acuity of the deviating eye to the initially low amblyopic level. Infants undergoing less severe patching regimens (e.g., one hour/day or two hours/day) display similar acuity trade-offs between the eyes. The acuity fluctuations in these cases, however, take longer periods of time to show effects as dramatic as constant occlusion. Our results, to date, indicate that the human visual nervous system, during the first year of life, is highly susceptible to modification by esotropia and short periods of occlusion. This research has obvious clinical applications for the monitoring and management of early strabismic amblyopia.